I think you will have access as per OW's website;

"Customers flying First or Business Class have access to the equivalent class of lounge regardless of their frequent flyer status. (First Class passengers may be accompanied by one guest travelling on a flight operated and marketed by a oneworld airline).

Customers flying in Economy class on the domestic or intra-European segment to connect to or from a oneworld longhaul First or Business Class flight can also use the lounge applicable to the class in which they are travelling the longhaul segment.

Basically hang on to your boarding pass from LHR-HKG, print off this segment from the OW website (just in case) and you should be fine.